Sean C. Lema

Reproduction in fishes is sensitive to temperature. Elevated temperatures and anomalous ‘heat waves’ associated with climate change have the potential to impact fish reproductive performance and, in some cases, even induce sex reversals. Here we examine how thermal sensitivity in the hormone pathways regulating reproduction provides a framework for understanding impacts of warmer conditions on fish reproduction. Such effects will differ depending on evolved variation in temperature sensitivity of endocrine pathways regulating reproductive processes of sex determination/differentiation, gametogenesis and spawning, as well as how developmental timing of those processes varies with reproductive ecology. For fish populations unable to shift geographical range, persistence under future climates may require changes in temperature responsiveness of the hormone pathways regulating reproductive processes. How thermal sensitivity in those hormone pathways varies among populations and species, how those pathways generate temperature maxima for reproduction, and how rapidly reproductive thermal tolerances can change via adaptation or transgenerational plasticity will shape which fishes are most at risk for impaired reproduction under rising temperatures. This article is part of the theme issue ‘Endocrine responses to environmental variation: conceptual approaches and recent developments’.

Filter-feeding invertebrates such as bivalves have been shown to improve the health of aquatic systems by reducing concentrations of bacteria and other harmful suspended organisms, but it remains unknown if microphagous suspension-feeding fishes can provide similar ecosystem services for water quality. Here, we tested whether the presence of the filter-feeding larval ammocoete life-stage of Pacific lamprey (Entosphenus tridentatus) can reduce suspended concentrations of Escherichia coli bacteria. Aquaria containing either filter-feeding ammocoete lamprey larvae (1.5 fish·L⁻¹), lamprey macropthalmia juveniles (1.5 fish·L⁻¹) that do not suspension-feed, or no lamprey (control) were filled with water contaminated with E. coli bacteria and then monitored for 5 d for E. coli concentration changes in the water column. The presence of ammocoete larvae generated a significantly faster decline in E. coli abundance compared to aquaria containing either macropthalmia-stage lamprey or no fish, which showed similar E. coli concentration profiles over that 5 d period. A higher density of ammocoetes (4.3 fish·L⁻¹) resulted in a more rapid decline in E. coli compared to the lower 1.5 fish·L⁻¹ ammocoete density, further implying that ammocoetes augmented bacterial clearance. These observations provide evidence that filter-feeding larval ammocoetes of Pacific lamprey may help promote water-quality enhancement by reducing suspended bacterial concentrations.

The jack silverside (Atherinopsis californiensis), also referred to as jacksmelt, is a neotropical silverside fish that inhabits nearshore shallow waters of the California Current Ecosystem in the Northeast Pacific Ocean, ranging from the coast of Oregon, USA, in the north to as far south as Baja California, Mexico. This fish is the sole member of its genus and is a commonly taken species when hook-and-line fishing in pelagic-neritic environments including bays, estuaries, kelp forests, and along sand beaches. Here we report the first complete mitochondrial genome of jack silverside consisting of 16,519 bp nucleotides and encoding 13 protein-coding regions, 12S and 16S rRNAs, 22 tRNAs, and an 841 bp D-loop control region. Phylogenetic analysis using all protein-coding genes of the complete mitogenome confirmed the inclusion of A. californiensis within subfamily Atherinopsinae of family Atherinopsidae, order Atheriniformes. This complete mitochondrial DNA genome will be of use for biodiversity assessments in the California Current ecosystem, while also providing a foundation for future mtDNA population genetic studies on this prominently caught species in shore- and pier-based recreational sport fishing.